1. Field of the Invention
The present invention pertains to a new process for forming coherent particulate composites, as well as to the composite itself. This invention particularly relates to a molding pelletizing or extrusion process for forming high strength particulate composites using a urea-lignosulfonate binder. Such composites are useful, for example, for forming carbon electrodes used, inter alia, in aluminum refining.
2. Description of the Prior Art
In the standard procedure for preparing carbon electrodes, a mixture of powdered petroleum coke, powdered metallurgical coke and binder is molded at an elevated temperature into the desired shape. Coal tar pitch, i.e., the residue of coke oven tar distillation, is generally used as the binder. The so-formed electrode is then "fired" by heating in a relatively inert atmosphere, e.g., under a nitrogen purge, at a temperature generally above about 1000.degree. C. Such heating pyrolyzes the binder and gives the electrode a permanent form, with high mechanical strength and thermal conductivity and low electrical resistance.
Because of the immense size of electric furnace electrodes, diameters typically range between 14-57 inches with lengths between 5-15 feet, it is critical that the molded "green" electrode have sufficient strength to withstand the rigors of subsequent handling and processing. This is particularly important since each electrode requires a great deal of time and expense to produce. Electrode rejects and failures are very costly.
It is an object of the present invention to provide a new binder that can be used, for example, in forming furnace electrodes. Electrodes produced using this new binder exhibit much improved "green" strength. A key component in this new binder is the lignin derivative lignosulfonate.
Besides cellulose, lignin is the principal constituent of the woody structure of higher plants. Lignin acts as a cementing agent in plants binding cellulose fibers together in a rigid structure. Commercially, lignin is available in the form of lignosulfonates and has been used in the prior art as a binder, filler and resin extender. The primary source of lignosulfonates is the pulp industry.
Woddrop U.S. Pat. No. 1,221,259; Tanberg U.S. Pat. No. 2,495,148; Seymour U.S. Pat. No. 3,635,684; and Blackmore U.S. Pat. No. 3,725,029 all disclose using lignosulfonates as at least one component of a binder composition in forming particulate composites. In Woddrop, for example, a binder composition including lignosulfonate powder, powdered hard coal tar pitch and fine powdered coal is disclosed. The binder is blended with a granular material, e.g., sand, to form a mixture which is then moistened, molded and dried at an elevated temperature, e.g., 350.degree. F., to form a composite. The composite can be used, for example, as a mold in forming metal castings.
In Tanberg, a carbon electrode is formed by blending, molding and drying a mixture of crushed anthracite coal or coke and a minor fraction (i.e., 10-35% by weight) of a concentrated aqueous solution of lignosulfonate. Tanberg notes that the binder can be carbonized at a relatively low temperature, i.e., 200.degree.-300.degree. C., making it an ideal binder for the continuous process for preparing electrodes.
In Seymour, a composite fuel log is formed by blending and molding a mixture of ground corn cobs, including husk material, parafin wax, liquid lignosulfonate and starch.
Finally, in Blackmore a composite ammonium sulfate fertilizer suitable for dry blending with other particulate fertilizers is formed by blending powdered ammonium sulfate with a concentrated aqueous solution of lignosulfonates. The ammonium sulfate is heated (90.degree. to 120.degree. C.), granulated with the concentrated lignosulfonate binder and cooled. The ammonium sulfate granules can then be provided with a urea coating.
It is an object of this invention to provide an improved process for binding particulate materials into coherent composites.
It is also an object of this invention to provide a high strength composite formed by binding particulate materials with a lignosulfonate-containing binder.
It is another object of this invention to provide a urea-lignosulfonate binder composition useful for forming high strength particulate composites.
It is a further object of this invention to provide a process for forming high strength particulate composites useful as carbon electrodes.
Other objects and advantages of this invention will become apparent from the following disclosure.